With good characteristics in terms of heat resistance, chemical resistance, and mechanical characteristics, electrical insulating sheets, such as polyester film, in recent years, have been used in many fields including magnetic recording material, various types of photographic material, electrical insulation material, and various types of release paper. In some cases, special surface characteristics are required for specific uses, and various coating layers (coated film) are provided on a surface of such sheets to meet these requirements. For example, coating liquids including magnetic coatings, ink coatings, lubricating coatings, releasing coatings, and hard coatings are applied over the surface of the sheets to produce a thin coat layer of these materials.
Coating apparatuses that comprise a coating bar, gravure roll, die, etc. are known as means to apply a coating liquid on a surface of electrical insulating sheets that are traveling continuously. In these coating apparatuses, a coating liquid is applied to a surface of the sheet while being weighed so that the thickness of the coating liquid applied to the traveling sheet will be in a predetermined range. In a coating bar type apparatus, for example, a coating liquid is supplied onto the traveling sheet, and the amount of the coating liquid is measured with a coating bar while removing the excess liquid, followed by smoothing the surface to adjust the thickness to a predetermined range. In such coating liquid application processes, a “liquid pool” of the coating liquid, which can be small or large as the case may be, forms in the neighborhood of the coating or smoothing means. For example, such a “liquid pool” can form in the gap between the coating bar and a portion of the surface of the sheet on the upstream side from the coating bar.
The liquid pool is also called meniscus. The shape of a liquid pool relates to the viscosity and the surface tension of the coating liquid. When a coating liquid is applied to a sheet, there will be an optimum range for the shape of the sheet (in terms of size, uniformity in the sheet's width direction, etc.). If a force is exerted to the liquid pool in the sheet's traveling direction, it will allow the coating liquid to be applied more uniformly on the sheet as a result of the coating liquid's adhesion to the sheet.
If the liquid pool is in an unstable state, however, some portions in the surface of the sheet may be left uncoated with the coating liquid, or streak-like defects may result from uneven coating in some cases. When using a coating bar, for example, it will be difficult to apply a coating liquid having a relatively high viscosity on an electrical insulating sheet.
Such an unstable state of the liquid pool can result from electrification of the electrical insulating coating liquid. In a charged coating liquid, each particle that constitutes the coating liquid is charged, and all these particles have the same polarity when charged. Thus a Coulomb's force is generated among the particles, resulting in repulsion among them. If there is a large repulsive Coulomb's force, a bumping-like state will be caused in the coating liquid in the liquid pool, making the shape of the liquid pool unstable. When particles repulse each other, furthermore, air will be easily taken in the coating liquid, making the liquid pool's shape more unstable. If the coating of the surface of the sheet with the coating liquid is performed when the liquid pool's shape is in such an unstable state, the coating liquid applied will not have a uniform thickness, resulting in uneven coating with the coating liquid.
As a conventional means of liquid pool stabilization for prevention of uneven coating with a coating liquid, a space containing the liquid pool is closed for isolation from the outside and depressurized to maintain the liquid pool in a stable shape. However, a coating apparatus that contains such a vacuum space will need a complicated mechanical structure and will have to be very large in size. In particular, such a coating apparatus is not suited for “in-line” use, i.e., incorporation in a process in which thermoplastic resin is melted and processed into a film which is then stretched to produce an electrical insulating sheet.
Such a liquid pool of a coating liquid that forms during the application of the coating liquid is described above. To the best of the inventors' knowledge, there are no conventional methods that can stabilize a liquid pool by using the Coulomb's force actively while controlling the amount of quantity of charge of the coating liquid.
On the other hand, the following methods are conventionally known as coating methods that comprise electrification of or static elimination from an electrical insulating sheet.
A first type coating method: When a coating liquid is supplied continuously on a traveling sheet to produce a coating layer, electrification of the sheet is performed immediately before a coating to accelerate adhesion of the coating liquid to the sheet.
A second type coating method: When a coating liquid is supplied continuously on a traveling sheet to produce a coating layer, static elimination from the sheet is performed immediately before a coating to ensure that disturbance of adhesion of the coating liquid to the sheet is prevented to control uneven coating.
The first type coating method, which aims to improve the wettability of a surface of an electrical insulating sheet, is disclosed in Patent document 1 or Patent document 2. Specifically, the first type coating method uses conventionally known corona discharge treatment to introduce polar functional groups in the coated surface of the electrical insulating sheet in order to ensure an optimum wettability by increasing the surface tension of the electrical insulating sheet. Thus, the electrical insulating sheet will be charged at the same time, resulting in the phenomenon of sheet's electrification.
An apparatus for the corona discharge treatment comprises a discharging electrode such as a wire that is provided on the sheet's coated surface side to cause corona discharge, and an grounding roll that is contact with the sheet's surface opposite to the coated surface to support the traveling of the sheet. The grounding roll acts as shield electrode (or grounded electrode, grounded counter electrode) to assist the corona discharge, adjusting the potential on the rear side of the electrical insulating sheet to 0V. The configuration of the corona discharge treatment apparatus is disclosed in Patent document 2.
FIG. 15 gives a schematic side view of a coating apparatus 150 having a corona discharge treatment apparatus 151 that is disclosed in Patent document 2. In FIG. 15, the coating apparatus 150 has the corona discharge treatment apparatus 151 and a coating liquid supply apparatus 155 in the direction from the upstream side to the downstream side in a traveling direction PSD of an electrical insulating sheet PS. The coating liquid supply apparatus 155 includes a discharging means 156 for the coating liquid PC and a pump 157 for supplying a coating liquid PC to the discharging means 156. Opposed to the discharging means 156, there is a backup roll 158 that is in contact with a surface PS2 of the sheet PS that is opposite to a coating surface PS1. The coating liquid PC is discharged from the discharging means 156 to the coating surface PS1 of the sheet PS, and applied to the coating surface PS1. A liquid pool PCP of the coating liquid PC is formed between the coating surface PS1 and the discharging means 156. The coating liquid PC which has been applied to the coating surface PS1 forms a coating layer PCL on the coating surface PS1.
The corona discharge treatment apparatus 151 comprises a grounded counter electrode roll 152 that is in contact with the surface PS2 opposite to the coating surface PS1 of the sheet PS, and a corona discharge electrode 153 that is opposed to the grounded counter electrode roll 152 and situated on the side of the coating surface PS1 of the sheet PS with a gap from the coating surface PS1. The corona discharge electrode 153 is connected to a corona discharge treatment power supply 154.
The electrical insulating sheet PS is kept in contact with the grounded counter electrode roll 152 as it is conveyed in the traveling direction PSD. The grounded counter electrode roll 152 serves to allow the opposite surface PS2 to the coating surface PS1 of the sheet PS to have a potential of 0V. As the sheet PS passes between the corona discharge electrode 153 and the grounded counter electrode roll 152, the sheet PS is exposed to a corona discharge space that contains a large amount of ions and radicals, and polar functional groups are introduced in the surface of the sheet PS. During this process, the sheet PS is charged at the same time. This treatment allows the surface of the sheet PS to have a high wettability, and the attractive force generated by the electrostatic charge serves to enhance adhesion of the coating liquid PC to the sheet PS.
Patent document 2 has no concrete description about the polarity of the charged coating liquid, but the proposed method uses the attractive force resulting from electrification, suggesting that the method only aims to allow the electrical insulating sheet PS and the coating liquid PC to have opposite polarities, or respectively have a potential of 0V and either positive or negative polarity, to cause an attracting Coulomb's force. To the best of the inventors' knowledge, this method cannot serve to stabilize the shape of the liquid pool PCP.
The second type coating method, on the other hand, is disclosed in Patent document 3 or Patent document 4. The conventional method, however, cannot perform sufficient static elimination from the surfaces of the electrical insulating sheet, and therefore cannot eliminate uneven coating resulting from the electrification of the electrical insulating sheet.
A static elimination apparatuses based on conventional technology uses a static eliminator that uses the generally known corona discharge. Such static eliminators include self-discharge type static eliminators in which a grounded brush-like conductor comes close to an charged electrical insulating sheet so that corona discharge will take place at the end of the brush to achieve static elimination; and alternating current type and direct current type voltage-applying static eliminators in which a commercial-frequency high voltage or a direct-current high voltage is applied to a needle-like electrode to cause corona discharge which is used for static elimination. Conventional static elimination methods that use corona discharge are designed to allow the resulting positive and negative ions to be attracted by the Coulomb's force caused by the positive and negative charges on the electrical insulating sheet, followed by equilibration with the positive and negative charges to achieve the neutralization of the charges on the sheet.
However, if positively and negatively charged portions coexist, close to each other, on the electrical insulating sheet, the electric force lines formed by the charges will be closed among the charged portions with opposite polarities. At somewhat distant portions, therefore, the intensity of the electric field will be so small that it will be impossible to attract necessary ions from the static eliminator, making it difficult to eliminate positive and negative electrostatic charges from the sheet.
Similarly, if the two surfaces of the electrical insulating sheet have opposite polarities, making the sheet apparently non-charged, the electric force lines formed by the electrostatic charges will be closed among the oppositely charged portions existing on the opposite surfaces of the sheet. Therefore, it will be impossible to attract necessary ions from the static eliminator. Conventional static eliminators cannot be useful for static elimination before coating if positively and negatively charged portions coexist. For a charged sheet having the two surfaces charged oppositely to the equivalent degree, it was impossible to achieve sufficient static elimination from these surfaces, failing to prevent uneven coating completely.
For control of the charge of the coating liquid, on the other hand, Patent document 5 has disclosed a method in which a charge feed apparatus is provided in the coating liquid supply pipe to remove static charges from the charged coating liquid or to charge the coating liquid oppositely before coating. FIG. 16 schematically shows a longitudinal section that describes the technique disclosed in the Patent document 5.
In FIG. 16, a charge feed apparatus 161 comprises a coating liquid supply pipe 162a, an electrode tube 164 provided via an insulator 163 with a gap from the coating liquid supply pipe 162a, and an insulator 165 provided outside the electrode tube 164 to protect the electrode tube 164, and a high voltage power supply 166. The coating liquid supply pipe 162a constitutes a part of a coating liquid supply piping 162. A high voltage is applied to the electrode tube 164 from the high voltage power supply 166. A high voltage of 4 kV is applied to the electrode tube 164 in an example given in Patent document 5. A charge is induced in the coating liquid supply pipe 162a when a high voltage is applied to the electrode tube 164.
In this electrification apparatus, however, the electrode tube 45 is not in contact with the coating liquid, making it impossible to feed a charge into the coating liquid. So, the amount of quantity of charge of the coating liquid is low though a high voltage is applied. It was impossible to charge the coating liquid to a sufficient level at a low coating liquid flow rate although it was possible to remove the charge from the charged coating liquid. This indicates that the charge feed apparatus 161 disclosed in Patent document 5 is not an efficient electrification apparatus for electrification of an electrical insulating coating liquid when used in equipment designed to coat a continuously traveling electrical insulating sheet.
As described above, no techniques are currently available that can stabilize a liquid pool easily without making the coating apparatus complicated, and conventional techniques cannot eliminate the problem of uneven coating resulting from the instability of the liquid pool. In particular, the liquid pool tends to be unstable, making the coating uneven, if the coating liquid is an electrical insulating liquid having a high viscosity. In addition, electrification of the electrical insulating sheet also acts to cause uneven coating. It has been impossible, furthermore, to supply an electrical insulating coating liquid to a coating apparatus after efficiently charging the liquid.                Patent document 1: JP 11-128804 A        Patent document 2: JP 2597237 B        Patent document 3: JP 10-259328 A        Patent document 4: JP 2817056 B        Patent document 5: JP 9-253565 A        Patent document 6: JP 2004-39421 A        Patent document 7: US 2005/0030694 A1        Non-patent document 1: Electrostatics Handbook; ed. Institute of Electrostatics Japan; pub. Ohmsha, Ltd.; p. 319        Non-patent document 2: Electrostatics Handbook; ed. Institute of Electrostatics Japan; pub. Ohmsha, Ltd.; p. 179        